A conventional spectrophotometer is proposed in the Japanese Published Unexamined Patent Application No. H6-34525 which can be used for a spectral analysis of a two-dimensional target area of a sample at high speed. FIG. 4 is a diagram showing the configuration of the optical system of the conventional spectrophotometer. When a rod-shaped light source (10) throws light on a sample (11), the light illuminates uniformly around a linear target section to be measured extending along the Y axis on the sample (11). Light from the linear target section passes through a lens (12) and is converged onto a slit (13). The light passing through the slit (13) is projected onto a diffraction grating (14), on which an image of the linear target section is formed. The light is dispersed with respect to the wavelength by the diffraction grating (14), where the direction of the dispersion is perpendicular to the length of the image of the linear target section. The dispersed light is reflected by a mirror (15) and arrives at a photo-detector (16), on which a two-dimensional image (which is referred to as a "spacial-chromatic image" ) is formed. One dimension (which will be referred to as the second dimension later) of the spacial-chromatic image corresponds to the length of the linear target section on the sample (11) and the other dimension (which will be referred to as the third dimension later) corresponds to the wavelength of the chromatic spectrum of each point in the linear target section. The photo-detector (16) consists of a large number of photocells arrayed in two dimensions corresponding to the two dimensions cited above. Thus the photo-detector (16) detects at a time the chromatic spectra (where a chromatic spectrum is a distribution of intensity with respect to the wavelength) of all the points in the linear target section on the sample (11).
The sample (11) is placed on a stage (not shown) which is movable along the direction (X direction) perpendicular to the length of the linear target section (Y direction). The stage is moved step by step in the X direction plurality of times, while in each step the chromatic spectra of all the points in the linear target section are measured as described above, until the whole two-dimensional target area of the sample (11) is swept. The distance moved by the sample (11) corresponds to the first dimension.
The spacial resolving power of the above apparatus in the Y direction, or in the length of the linear target section, on the sample (11), which corresponds to the longitudinal direction of the slit (13), depends on the number of the photocells included in the scope of the spacial-chromatic image projected on the photo-detector (16) in the second dimension. On the other hand, the spacial resolving power in the X direction, or in the dimension perpendicular to the length of the linear target section of the sample (11), which corresponds to the direction perpendicular to the longitudinal direction of the slit (13), depends mainly on the width of the slit (13). Practically, however, the size of the spacial-chromatic image projected on the photo-detector (16) depends on the configuration of the optical system, and besides, there is only limited choice for the size of the photocells practically used. Therefore the degree of freedom is low in determining the number of photocells in the scope of the projected image. Besides, the degree of freedom in determining the width of the slit is also low since it is significantly related to the chromatic (or wavelength) resolving power. For such reasons, it is difficult to design an apparatus whose resolving power in the X direction on the sample (11) is equal to that in the Y direction. Such a difference in the resolving power between the two directions on the sample (11), however, causes a problem that measurement results come to differ depending on how a sample is placed with respect to the longitudinal direction of the slit even when the same area of the same sample is measured, so that the reliability and reproducibility of measurement deteriorates.
Thus, a spectrophotometer for spectral analysis of a two-dimensional target area of a sample has been demanded by which the same result can be obtained from the same sample irrespective of how the sample is placed.